For fabricating a semiconductor device, such as an IC package, a semiconductor chip is mounted on a metal frame, which is called a lead frame. After mounting of the semiconductor chip on the metal frame, the semiconductor chip is wire-bonded to leads of the lead frame, and is entirely molded with resin. In general, such a lead frame is formed in a desirable shape by removing unnecessary portions with chemical etching technique or mechanical punching technique. Thus formed lead frame includes a die pad on which the semiconductor chip is mounted and support bars which support the die pad in a molding process. The support bars extend outwardly from the corners of the die pad in a straight line, respectively.
When the semiconductor chip is wire-bonded to leads of the lead frame, the position of each lead is automatically recognized on the basis of the position of the support bars. For better wire-bonding, the lead frame is heated to about one hundred and fifty degrees to two hundred and fifty degrees of Celsius' scale. When the lead frame is maintained at such a high temperature, the support bars are deformed with temperature. As a result, an automatic wire-bonding machine can not recognize the leads, because the support bars, which should be the basis of the recognition, are not at the initial positions anymore.
In a resin molding process, according to a conventional technology, it is difficult to provide the same amount of resin both on the upper and lower surfaces of the lead frame. In more detail, more resin is provided on the upper surface of the support bars than the lower surface thereof, because the same thickness of resin is usually intended to be provided on the upper surface of the semiconductor chip and the bottom surface of the die pad. Because of the difference of coefficients of thermal expansion between the resin and the lead frame, and of the difference of amount of resin arranged on the upper and lower surfaces of the lead frame, the semiconductor apparatus may warp when its temperature falls down to the room temperature.
A kind of semiconductor device is fabricated using a lead frame which is depressed at some portions to allow a die pad to be lowered relative to the other portion of the lead frame. In a die-bonding process, the semiconductor chip is mounted on the lowered die pad with die-bonding material, and the entire structure is heated to harden the die-bonding material. In the thermosetting process, the depressed portions of the lead frame are deformed by stress relaxation, so that the level difference at each of the depressed portions changes from its initial value. And therefore, according to the conventional technology, the die pad and/or the semiconductor chip is not well covered with resin in the molding process. As a result, the die pad and/or the semiconductor chip may be exposed to outside of a resin package.
When the semiconductor device is mounted on a substrate, the semiconductor device is heated to the melting temperature of a conductive paste. According to the conventional technology, the die pad is applied with some stress at its corners in the mounting process, so that some cracks may be made in the resin package.